Isolate and characterize T lymphoma cell and normal T cell receptors for T-MuLV's, and define their specificity in terms of the molecules and determinants that they recognize, as well as define whether they belong to a single class of cell surface receptors or several different kinds of receptors. We shall use techniques used for the isolation of cell surface receptors for defined ligands such as antigens (the receptors are immunoglobulins), hormones, neurotransmitters, and specific cell-surface antigens. Since the fibrotropic retroviruses in chickens and mice bind to fibroblasts by a viral envelop protein-cell surface receptor interaction, both lymphoid and fibroblast receptors will be isolated. We propose that the cell-surface receptors in fibroblasts are absent from most, if not all, thymocytes and thymic lymphoma cells; a test of this hypothesis will require an extensive comparison of type-specific and general receptors for the same ligands--the T-MuLV envelop glycoproteins (18-21).